An implanted cardiac pacemaker is coupled to one or two implantable stimulation leads which are implanted in one or both chambers of a patient's heart, respectively. Physical contractions of the heart muscle tissue generate low amplitude electrical signals (e.g., P-waves or R-waves) which can be monitored to obtain indications of the physical condition of the heart, and in particular to provide a basis for determining whether stimulation pulses must be supplied to the heart from the pacemaker. The implantable stimulation leads may be either unipolar, having a single tip electrode at the distal end of the lead, or bipolar, having a tip and ring electrode at the distal end of the lead. In the unipolar configuration, the pacemaker case acts like the return electrode. Thus, the stimulation pulses may be delivered either tip-to-case (unipolar) or tip-to-ring (bipolar).
A sensing module is typically coupled to the electrodes and is programmable between unipolar sensing and bipolar sensing modes. A pulse generator generates electrical stimulation pulses as a function of the status of cardiac signals supplied to the sensing module.
Proper sensing of low level intracardiac signals (e.g., P. waves and R-waves) depends on the fidelity, and accuracy, with which the these low level signals are amplified. In addition, implantable medical devices, such as cardiac pacemakers, operate with a self-contained power source that must have a long useful life, primarily when replacement of the power source requires a surgical procedure.
Proposals to satisfy these diverse, and somewhat contradictory, requirements have involved a wide variety of amplifier types. In particular, amplifiers of the continuous, linear type offer excellent signal amplification quality at the price of relatively high current consumption. On the other hand, a class of amplifiers typified by switched-capacitor amplifiers offer relatively low current consumption but introduces a significant noise component into the amplified signal, this noise component being due essentially to the turning off of switches associated with the capacitors. It therefore follows that each of these amplifiers has shortcomings with respect to one system requirement or another.